Alicia Schist

The Alicia Schist which appears to be Cretaceous, is the lowest formation  in  Bohol  area and the Ubay  Volcanics  which appear to be  Paleocene approximately 100-145 million years old are exposed  in  the eastern   to northern Bohol. The entire basement complex is overlain unconformably  by  a 2000 m thick sequence   of  Lower Miocene to Pleistocene carbonate and clastic sedimentary rocks and igneous units.

Brgy. Cayacay, Alicia, Bohol

1 hour & 53 minutes travel

HISTORY

The oldest rock unit in the island of Bohol is the elongated body of foliated rocks outcropping in the town of Alicia. It was called Alicia Schist since the rocks is mainly exposed at the town of the same name, located northeastern part of the island. It is a formation consisting of schists with three mineral compositions that grades from chlorite schists, quartz-sericite schist to amphibolite schist (Faustino et al., 2000). Difference in their color makes the schist distinguishable to each other – chlorite schist is olive green in color, the quartz-sericite schist is rusty brown to pinkish in appearance while the amphibolite schist appears greenish gray to dark gray in color. It is overlain by the Ubay Formation. Schist exposure extends for 15 km in a NS direction and is about 5 km wide. The Maubid Amphibolite of UNDP (1987) in the northwest is considered by MGB (2004) as part of the Alicia Schist. It is thrusted by the Cansiwang Melange of Faustino et al (2003) near Alicia. 

The Alicia Schist includes chlorite schist, quartz-sericite schist and amphibolite. The NW-trending, NE-NW dipping lineation in the rock body indicates the direction of the foliation. This is described as schistosity, the parallel alignment of platy mineral grains (muscovite and biotite) in a coarse-grained metamorphic rock. The schistose texture observed in Alicia Schist indicates that its protolith has been subjected to strong tectonic stresses that transformed its original texture and composition. The schist is produced by a regional metamorphic event related to extreme change in temperature and pressure. It originated as a continental crust fragment of a subduction-related marginal basin that had undergone ocean-floor regional metamorphism during the Mesozoic (Faustino et al., 2003) and was not formed as a consequence of the emplacement of the ophiolite. The schist has a “backarc basin geochemical signature with a subtle subduction affinity” (Yumul, et al, 2001).

Together with the Cretaceous to Paleocene Bohol Ophiolite and Cansiwang Melange, the three units make up the island’s basement complex. Cretaceous-Paleocene age was assigned based from its structural and stratigraphic relationships with other lithologies since no fossils were found. Alicia Schist is overlain by Paleocene – Late Eocene Ubay Formation which is composed of heterogenous mass of volcanic flows.

The exposure to be designated as a geosite is located at Brgy. Cayacay, Alicia, Bohol (9°54’14’’ N, 124°27’28’’E). This wall of rock is 30 meters wide and 8 meters high. Up close, greenish gray to dark gray color can be observed. Foliations can be seen as the banded appearance of the rock surface. The schist appears to be brittle. It was accessed via a rough road that branches off from the main highway along Alicia town.

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